This invention relates to phase shift photomask blanks in photolithography with short wavelength (i.e., &lt;400 nm) light. More specifically this invention relates to phase shift photomask blanks that attenuate and change the phase of transmitted light by 180.degree. relative to light propagating the same path length in air. Such photomask blanks are commonly known in the art as attenuating (embedded) phase shift photomask blanks or half-tone phase shift photomask blanks.
The electronics industry seeks to extend optical lithography for manufacture of high density integrated circuits to critical dimensions of 0.25 mm and smaller. To achieve this, lithographic photomask blanks will need to work with short wavelength light, i.e. &lt;400 nm. Two wavelengths targeted for future optical lithography are 248 nm (KrF laser wavelength) and 193 nm (ArF laser wavelength). A phase shift photomask enhances the patterned contrast of small circuit features by destructive optical interference.
The concept of a phase shift photomask and photomask blank that attenuates light and changes its phase was revealed by H. I. Smith in U.S. Pat. No. 4,890,309 ("Lithography Mask with a p-Phase Shifting Attenuator"). Known attenuating embedded phase shift photomask blanks fall mainly into two categories: (1) Cr-based photomask blanks containing Cr, Cr-oxide, Cr-carbide, Cr-nitride, Cr-fluoride or combinations thereof; and (2) SiO.sub.2 - or Si.sub.3 N.sub.4 -based photomask blanks, containing SiO.sub.2 or Si.sub.3 N.sub.4 together with a predominantly opaque material, such as MoN or MoSi.sub.2. Commonly the latter materials are referred to generically as `MoOSiN`.
Cr-based photomask blanks have the advantage that they are chemically durable and can use most of the familiar processing steps developed for opaque Cr photomask blanks. The second category of photomask blanks based on SiO.sub.2 - or Si.sub.3 N.sub.4 exploit their transparency into the deep UV and ease of dry etching with more innocuous fluorine-base chemistry. However, the need to develop photomask blanks for even shorter wavelengths (&lt;200 nm) renders Cr chemistries less desirable because photomask blanks based exclusively on Cr (i.e., oxides, nitrides, carbides, fluorides or combinations thereof) are too optically absorbing at such wavelengths. The disadvantage of `MoSiON` photomask blanks in this short wavelength regime is that they are too Si-rich and consequently have poor etch selectivity relative to the quartz (SiO.sub.2) substrate. Thus, they require an etch stop, an additional layer of a material which etches poorly in a fluorine etchant.
In addition, there are references in the literature to attenuating embedded phase shift photomask blanks comprising hydrogenated amorphous carbon layers, tantalum and its compounds with a layer of Cr metal, or one or more layers composed of a hafnium compound.